Revert "unfiltered display of preview"

This reverts commit 54502968799e58fe6567ab1c3d5cd81967a5cf7c.
I don't know, why the code was changed before, but that caused, that images
in menu weren't transparent, for example image of favourites in
the serverbrowser.
keep the last five versions of saved.cfg and init.cfg

Renames the existing saved.cfg to saved-old-1.cfg (and an existing -1 to
-2, and so on) before the new saved.cfg is written.

This should help in cases where players mess up their config, because the
config is deleted with a five-game-starts delay.
Fix gonext command and do it random

Commit fixes broken "gonext" command, among other things reading time
from maprot, and it does random result of this command.
It also removes get_next() function, because it messes up the maprot,
so shouldn't be used.
Keep selected player in op menus, when other player (dis)connects

Until now, when in, for example, kick player menu there was selected
some player and (dis)connected other player with lower CN, then this
earlier selected player was moved to other item in menu, but number
of selected item didn't change, so other player was selected.
Test work - ignore player menu
Add AssaultCube's CodeBlocks project for Linux
Don't show HUD reflection on water surface

For reflectclip > 1 player's HUD reflects on water surface.
Optimize and fix autodownload feature

The commit:
1. fixes value of download speed in KB;
2. introduces unique names of progress_callback() functions;
3. removes one unnecessary write_callback() function;
4. removes duplicated code/info in loadmodel().
Add custom description of demos

Custom description is displayed in "Watching demo" and "Play demo" menu.
Improve vote info

The commit:
1. adds additional small vote result for hidevote < 2, visible below radar,
2. sets default value of hidevote to 1,
3. shows final vote result in console
4. binds to H key toggling between hidevote 1/2 and 0.
Add altering pages in serverbrowser and improve favourites

Altering pages in serverbrowser are implemented with use favourites.
Categories (names) of favourite servers, which will be displayed on the
pages, have to be included in "serverbrowseraltpages" alias (accessible
in menu).
Switching between pages is possible with use ALT + mouse wheel or
"[", "]" keys.

The commit also:
1. adds MATCH category to favourites;
2. shows category name in settings of favourites;
3. shows weights in serverbrowser instead of favourites image,
when showweights = 1;
4. cleans up some tabs.