Added features to full metadata editor.

* Added conflict filter.
* Added drag 'n' drop into reqs, incs and load after list when conflict
filter is enabled.
* Combined "user edits enabled" checkbox and plugin name text controls.
* Added a column to the plugins list which displays ticks or crosses for
plugins with user metadata that is enabled or disabled.
* Removed bolding of font for plugin list entries with user metadata,
since that's handled by the new column now.

Everything seems to be working correctly.
Added priority columns to full editor plugin list.

Also slight code tidy-up.
Tweaked full editor window sizing.
Added simple display mode for metadata editor.

It gives the full editor an interface very similar to that of the mini
editor. The idea is that it will replace the mini editor, but the
sorting workflow needs to be adjusted slightly for that to happen.
Started work on pure HTML/CSS/JS UI.

I will probably end up using templating libraries and frameworks for it,
but for now getting stuck in learning how to do it myself.
A bit of styling and content generation.

The layout of the main view is now roughly correct.
Drop-down menus in top bar implemented.

Menu items don't do anything yet.
Started to implement some dialog display.