Implement persisting search history for Find in Files

Summary:
Major Changes:

• grep job settings are saved in the GrepDialog config sections
• when starting a session, grep jobs with the recalled settings are started

Minor Changes:

• refresh button now restarts grep job for currently selected result
• fix clearing of old entries beyond display limit in result view
• disable refresh and clear buttons for empty result history
• do not set up UI of GrepDialog when it stays hidden
• keep track of all GrepDialogs for plugin unloading (fix)
• fix saving of not yet inserted combo box entries

Remark:
To make sure all projects are loaded after startup (for searches
in project files only), the number of loaded projects must match
the number of projects to be loaded from the "General Options"
config section.

BUG: 380864
BUG: 381754

Test Plan:

• start a session with several projects to be opened
• use the "Find/Replace in Files" tool to search in files
• restart the same session and observe how the old searches are done after all projects have been loaded
• use the refresh button to redo a selected search

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6262