exporting ftml from server does not enforce/check ftml ending

[Joern Ungermann]

Extension should be checked/enforced similar to “save as” feature of MSS.

[Reimar Bauer]

Force ftml file type when saving and minor fixes

• No error message when using "cancel" in dialogue
• use save/open shims as in mss main menu
• force ftml file type

fix issue #487

→ <<cset e20d489715bb>>