Added: command 'make screenshot'

Now one may gradually remove 'make screenshot'-duplicates from other Lua-modules.
TODO: Since Shift+P is in use in inputbox to enter char 'P', one should discuss replacing Shift+P by something else, that is still free. Just an info: inputbox uses all buttons without modifiers => small characters; all buttons with Shift => CAPITALS; Alt+Q..P = digits (0..9) and etc. 
Next tigran's requests

1. Finally, removed infomessage about redrawing page in new zoom mode
2. Prevented potentially wrong user's input of non-integer values for page numbers in the 'go to page' input box
3. Removed duplicated code to make screenshot; already included in commands.lua
Add functions to change font face for all items in Font.fontmap

1. Excluded pgfont from Font.fontmap-items; it was used in helppage.lua to render footer and just duplicated ffont
2. Fixed typo: self.cfont > "cfont" in Font:getFace()
3. 3 small functions to change font face for any member of Font.fontmap
Improved the method to define font face to render menu items

1. Add parameter own_glyph that defines how to render menu items:
default own_glyph = 0,	-- render menu items with default "cfont"
own_glyph = 1 => own glyphs for items like "Droid/DroidSans.ttf"
own_glyph = 2 => own glyphs for Font.fontmap._index like "ffont", "tfont", etc.
Now own font face may be used to render both types of menu items called from Font:chooseFonts() 
2. Delete some outdated comments
Clear the code from not used parameters

1. Remove a lot of unused self parameters & commands (including 'make screenshot') 
2. Make sure adding commands only once
3. Add function to change font faces for any items enumerated in Fonts.fontmap and modify the code to recalculate the width of left column that depends on the selected font face => lface
4. Remove most of outdated comments
Replace old 'Font menu'-function by new, improved version 

1. Replace old function to change 'cfont' by new, improved version capable to change all font faces.
2. Remove 'make screenshot'; already added in commands.lua
2. Moved exit hotkeys (Back & Home) to the end of the hotkey list
3. Remove some outdated comments and clean a bit the code (just cosmetics to make it more readable)
Replaced: font selection menu (new version), etc

1. New menu Font:chooseFonts() to change font faces
2. Removed duplicate of 'make screenshot'
3. Hotkeys (Back & Home) to exit are moved down, to the end of hotkey list
4. Removed outdated comments & not informative comments
Replaced: font-related 'self' parameters > local 

1. Made few (font-related) self parameters to be local > now updating font faces in Font:chooseFonts() properly changes these font faces
3. Removed outdated comments; minor code cleanup: to make it more readable
Message font: "cfont" > "infofont"

1. Removed useless parameter self.face
2. Changed font face from "cfont" to "infofont"
Add global parameter filemanager_expert_mode

In order to switch between safe and advanced way for filechooser to manage files, I've introduced special parameter 'filemanager_expert_mode':
i) 'safe' means that filemanager shows only files with the reader-related extentions; make safe renaming (i.e. the extention remains the same)
ii) 'advanced': all files are available (no filtering); any renaming is allowed;
TODO: to add also 'expert' mode & use it for unstable or dangerous functions
Restored proper separation of cre-documents

The decision whether ext:getReader(ftype) returns CREReader or nil for documents with improper file extentions now depends on the new parameter FileChooser.filemanager_expert_mode. Improper files are now related to CREReader only for advances users.
Enter filemanager modes for beginners & advanced users 

1. Added parameter that tells filemanager the way how to manage files. Those who prefer 'safe' method (default) should not select any other mode except for BEGINNERS_MODE in new menu called by Alt+M. Actually, 'safe mode' is our old version (the files are filtered on the extention-basis; one may rename the filenames, but not their extentions). The tigran's corrections https://github.com/hwhw/kindlepdfviewer/commit/de0f40ce9cf59f06658ea71d8d34248501273d81
are easy to switch on by selecting mode for advanced users who understand potential danger (mostly, crashes of crereader 'cause of improper input files) and accept it.
2. New menu Font:chooseFonts() to change font faces
3. Removed duplicated code of 'make screenshot'
4. The hotkeys (Back & Home) to exit FileChooser are moved down
5. Removed outdated comments
Merge pull request #251 from NuPogodi/master

New menu to change font faces; new parameter to handle files; etc