qt: improve default widths of advanced prefs panels

many panels in the advanced view had horizontal scrolling under the default widths, giving poor UX. a lot of space was unnecessarily eaten up by the tree, with width simply shared in a 50:50 split between the tree and the page panels. (counting just exactly how many is made difficult by the fact that some long panel titles force automatic width adjustments, and it's just not worth the effort to work around that to get a count).

with addition of a size policy we can very comfortably steal 100px from the tree, giving a 3:5 ratio. this alone makes a big difference, but still leaves 9 panels unfixed on my platform, including four rather significant ones (two root category nodes and two subcats). an additional bump of +50px to the panel width takes care of 3 of those.

of the remaining 6:

• 3 are due to a couple of filters with rather long names. a rather substantial further increase in panel width would be required to prevent the problem for these. it would be better to simplify the names.
• 2 are due to a long option name in the x264 plugins, which can be fixed later by removing an unnecessary portion of the shorttext.
• the splitters-clone case remains a little bit off. we could fix this with a further +25px maybe, however i plan to shorten certain names anyway in later work which will do the job without such extra width so it's perhaps not worth it.

this thus implements the size policy, and adjusts the tree and panel widths from 400px each to 300px and 550px respectively.

users can always adjust these manually, but better to improve the defaults!