Set POT File and Update From That

[ravage84]

Context:
While it is possible to set source paths and update a PO file from them it is not possible to set the (relative) path of the POT file.
Thus, one does always need to select the POT file manually

Feature request:
It would be nice to set the POT file for a PO file once and update from that without needing to select the POT file every time.

Rationale:
The path of a POT file most likely never changes and is most likey always the same (relative) to the PO file.
Also, selecting the wrong POT file leads to mismagled PO files, which is annoying.

[vslavik]

Duplicate of #79.