GZDB merging - part 1

A bunch of cherry-picked GZDB commits, from where ZB diverged (8 Feb 2016) to 8 Mar 2016, with the exception of parser changes as those are hard to incorporate with the SRB2-specific parsers. I'll try to incorporate those at some point, and rewrite the SRB2-specific parsers accordingly.

A summary of the changes:

• Fixed a crash with the testing dropdown menu, added warning icons for test exes that can't be found

• Various fixes to the texture browser

• Some optimizations & bugfixes for Visual Mode

• Continuous drawing for all drawing modes

• Searching for all lines with actions, by using -1 as an input

• Added "lock slices to grid" options to Draw Grid mode

• Added angle parameter to Draw Ellipse mode, lowered minimum subdivisions to 3