TSynBaseCompletionProposalForm styling

[vincentparrett]

See issue #224

Note there are still some high dpi issues with the proposal form, which stem from the fact that it is created dynamically and there is no dfm for it (so it calles CreateNew). This is a vcl issue nothing to do with this code. I'm still looking for work arounds for this.

What this PR addresses

• Flicker when scrolling - the paint routines have been optimised to avoid repainting the entire form during scrolling.
• Styling - now uses a single border rather than the thick border work around when resizable. Also uses the statusbar gripper from the style.
• Other minor tweaks and some dpi scaling fixes.

[pyscripter]

@vincentparrett Have you seen #239? If you submit your PR to https://github.com/pyscripter/SynEdit, I will have a closer look.

[vincentparrett]

I hadn't seen that, will do.