Fix several bugs related to wad file loading.

- Build absolute path from relative wad path by using the map file,
  executable, and Quake path as reference paths in that order.
- Properly update and save wad key when changing texture wads.
Fix camera direction and up becoming non-normal when trying to
rotate past +/-90 degrees (up / down).
Fix camera discarding horizontal adjustments when during orbiting
when view direction is equal to Z axis.
Fix a crash when logging a warning for invalid faces and improve
error message.
Use Shift+Click for face selection instead of clicking on a selected
brush.
Make the standard definition parser more forgiving when parsing
numbers.
Properly determine the end of an entity definition in the standard
definition parser.
Remove face highlight.
Use GL_NEAREST for texture minification and magnification filtering.
Add commands for increasing and decreasing the grid size.
Flush textures from video memory after clearing the map.
Allow the user to switch the move direction during a drag.
Flush entity and texture browser cached strings when reloading its
contents.