Read filetype's mime type from the configuration files

This allows custom filetypes to define a mime type, hence the icon
displayed for this filetype;  as well as moving ones for built-in
filetypes out of the code to the configuration files.
Use GIcon for the filetype icon rather than rendered pixbuf

This allows to re-use the icon more easily since it's not rendered at
a particular size but simply an icon description.  It also allows for
implicit icon updating when the theme changes if the display code
implements it (and GTK widgets does).
Update of Russian translation
Install header files and geany.pc also on Windows
Add a new section to the Windows installer to install header files
Add a warning about using plugin_signal_connect() on short-lived objects
Update of Portuguese translation
Updated Hungarian translation
Fix possible crash on quit

When quitting, we still have to destroy the Scintilla widget to avoid
any possibility for us to receive signals from it after we destroyed
the associated editor and/or document (used in signal handlers).

I myself don't suffer from the issue, but it is theoretically possible
for Scintilla to emit signals anytime before it is destroyed, so it is
safer like this anyway.  And an user on IRC suffered from crashes on
quit because of this issue, so it seems to actually happen in some
situations.
Fix a missing cast (oops)
Merge pull request #241 from kgkilo/master

Updated Hungarian translation
Update list of updated translation of NEWS file
Add --no-scm configure option to bypass SCM detection
Add linker flags -static-libgcc and -static-libstdc++ on Windows

This seems necessary with newer gcc versions to not
require libgcc*.dll libraries on runtime.
Add colorschemes from the geany-themes project if available
Fix a little typo on NEWS
Re-add the previous workaround to remember the directory Geany was started from

My previous fix (e1ce9cfbec) didn't fix all possible use cases (opening
files from CLI when starting Geany). This one should close the gap.
Update displayed copyright years
Update NEWS for missing translation updates
Add some missing NEWS items