Improve usability of find missing file dialog

[iromeo]

At the moment there are usability issues in this dialogue:

• Cannot copy file name, e.g. to find it using 3-rd party tool (Terminal / System search application / File manager)
• When trying to select path, always file open dialog is opened, which is confusing
• If I know new name, e.g. file ext changed from bed to bed3 than it is easy to type 3 at the end than file new file using ... button. E.g. if folder has 30 files it is very inconvenient to find smth especially when file name cannot by copy-pasted

[iromeo]

Improvements made:

• allow copying file name from missing track source dialogue to clipboard
• allow to edit path + validate name after user has pressed 'ENTER' in the text filed
• add default copy/paste/cut actions into text editor context menu