Add scratchpad plugin

Summary:
Adds a scratchpad plugin, which allows you to keep "scratches" of code/text
to experiment or quickly run something without the need for a project.
The plugin adds a new tool-view, which will maintain a list of your scratches
as well as allowing you to compile and run them.
The scratches live in the directory scratches in the data directory and
are regular documents so we get all the editing convenience of code-completion
and diagnostics.
Commands used to run them are saved per-scratches and new scratches use the last
used command for that file type/suffix.

FEATURE: 176389

Test Plan:
Add the tool-view on the left tool bar and try creating and using the scratches.
Currently no automated tests.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: aaronpuchert, kfunk, gregormi, brauch, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D16484