Save Tabs to Text: Keybind wót SRA

Allows for the use of a keybind to activate the save menu. This defaults to Ctrl+S, which likely conflicts with the Save Page As... keyboard shortcut, so you will either need to change it for the addon, or change the Save Page As... shortcut in about:keyboard.

This is quite minimal, as of yet; I hope to add features in the future. It is modified from Save Tabs to Text, which seems to have halted development.