Allow executing arbitrary commands from project.xml.

[player-03]

The <execute /> tag immediately runs a single command, and optionally saves its output and/or exit code. Sample usage:

<section unless="display">
	<execute command="script.bat" if="windows" exitCode="scriptExitCode" />
	<execute command="./script.sh" unless="windows" exitCode="scriptExitCode" />
	<error value="Custom script failed!" if="${scriptExitCode}!=0" />

	<execute command="dir" if="windows" output="directoryContents" />
	<execute command="ls" unless="windows" output="directoryContents" />
	<echo value="Directory contents: ${directoryContents}" />
</section>

Checking unless="display" is optional but recommended. Without it, the commands will run every time code completion is initialized.

Closes #1959