Little fixes for TextureBrowser's filter

Hi. I've made little improvements for TextureBrowser's filter.

1. Changed TextureBrowser filter to case-insensitive. (aa3d5bc)

2. TextureBrowser filter entry now won't loose focus after searching. (57fc5ec and 5ef4928)

3. Added incremental search and ability to search in names only (ignoring path). Requires all commits except the last two.

Last two commits are just maintenance.

I don't know if those features fit into DarkRadiant's ui philosophy, but the first one at least is rather helpful.
If you think the third feature is helpful but should be optional, I can add checkboxes in the UI for "Incremental search" and "Search in names only".