Fixes for ThingPropsPanel

- Actually save entered Z Height and TID
- Don't apply thing type 0 if no type set (multiple selection)
Merge branch 'master' of https://github.com/sirjuddington/SLADE.git
Initialise/copy thing type and action special arg count properly

May have been causing crashes in the thing properties panel
Rearrange/tweak start page

Should look better with older versions of IE (WinXP), and allows the recent files list to expand more in case of long filenames that get wrapped
More mior tweaks to start page
Various additional map editor colour configuration settings

- New ColourConfiguration 'theme' section, has settings for line hilight/selection width and (textured) flat alpha multiplier
- Updated/improved 'Clean' colour configuration
- New 'Doom Builder 2' colour configuration
Fix potential crash loading executables.cfg

Not sure how people are getting a blank one...
Don't draw direction arrows for square things
Minor additions/improvements to map preview canvas

- Moved colours to colour configuration
- Added simple things to preview
Move map image export colours to colour configuration
Open image file after saving map image
Add support for mouse4 and mouse5 keybinds
Load game configuration properly when creating a new map
Fix potential crash on menu action
Separate 3d info overlay colours from 2d config
Always take zoom into account when updating the hilight
 Add priority system to keybinds

Probably not completely needed, but is more of a workaround for problems that occur when multiple things have the same key binding (eg. 'accept' and 'edit object properties')
Show message box if errors occured on audio conversion
Fix problem with keybind press
Fix for gcc compilation