Select only filename without extension in rename dialog box

Summary:
Renaming a file in browse mode preselects the filename without extension.
In view mode the whole filename (including extension) is preselected.
This patch adds a custom dialog, which selects only the filename in view mode rename.

Test Plan:
1. Open image in view mode
2. Press {key F2} to rename file
-> Filename without extension should be selected

Reviewers: rkflx, ngraham

Reviewed By: rkflx, ngraham

Subscribers: anthonyfieroni

Differential Revision: https://phabricator.kde.org/D9632