Some fixes for the stash manager to make it more informative

Previously, the stash manager would just list a series of stash@{n} lines, which don't really give a lot of information about a given stash item:

Plus, as shown in the screenshot, the resizing the dialog led to a weird layout and an incorrect tooltip was shown for the "Drop" button. Also, when the Patch Review plugin was disabled, clicking on the "Show" button would crash kdevelop.

This merge request fixes these things, so that now the dialog shows more info, is resizable, and the "Show" button works even when Patch Review is not enabled (by opening the diff in an editor tab in that case):