Give the Script Editor an Install command #59

From @rasband:

Eclipse and NetBeans can be overwhelming to a scientist who wants to develop a simple plugin or who wants to install a plugin only available in source form. The Script Editor should have an Install command similar to the Plugins>Compile and Run command, which no longer works in Fiji. Fiji's Plugins>Install Plugin command is not the solution since the installed plugin appears to be compiled every time you run it.

See also discussion at Fiji bug #1074.