Merge branch 'Melanosuchus/texture_browser' into 'master'

Melanosuchus/texture_browser

Some improvements to the texture browser:

* "Fixed Size" enabled by default (makes it easier to select textures and looks nicer)
* Option (in the menu) to show the transparency (over the commonly used gray checkerboard, transparent images look much nicer)
* When shaders are refreshed, forces reloading the textures as well. Previously it would at best only display shaders, at worst mess up some textured objects.
* Option (in the preferences dialog) to change the texture thumbnail size when they are displayed with a fixed size

See merge request !3