Add xsize, ysize, and xysize style properties.

These allow users to directly set the size of resizable
displayables.

The names are chosen so they don't conflict with height, width, and
size, which are already in use.
Make the fonts used by A White Tulip customizable.
Update options.rpy
Update common.rpy
Update common.rpy
Update distribute.rpy
Update project.rpy
Update common.rpy
Update common.rpy
Update common.rpy
Use the unexpanded size of text when checking for overflows.

Fixes #286.
Report an error if an imagebutton can't find its idle image.
Merge pull request #287 from javimat/master

Update Spanish translation
Fix spanish translation.
Add new keywords to screenlang and sphinx.
The focus_mask style property can now as an argument a callable.

If the callable returns true, the displayable should be focused.
This lets us avoid an RTT.
Update changelog.
Return to the main menu on right click from load, save, prefs, etc.

Alternative implementation of #289.
Add Preference window during transitions
Remove window auto, adjust semantics of window during transitions.

This is separating the two functions out, since window during
transitions is useful with window show/window hide, and the
auto function should probably trigger window show/window hide.