Document LEKTOR_DEV and support it in GUI App

[item4]

This make plugin developing hard.

Reason is simple, lektor command reuse first loaded env, so non-document file was detected change by watchdog but happen nothing.

I want to reload plugins and configs automatically when I changed plugin or config.

[mitsuhiko]

LEKTOR_DEV=1 enables reloading for plugins. However this does not work with the GUI app yet.

[xlotlu]

This is now documented and there's no GUI app anymore.