Add `control` property to address a custom control via CLI

[waydabber]

This is required so one can access custom controls (DDC and CLI) created in the app.

Also works with #4135.

[waydabber]

Handle (custom) ranged controls. The set sets a value, get gets a value, feed overrides the control's value (for example from an external source) without invoking set as described in #4135.

betterdisplaycli get n=DISPLAYNAME -control -specifier=customRanged
betterdisplaycli set n=DISPLAYNAME -control=30% -specifier=customRanged
betterdisplaycli feed n=DISPLAYNAME -control=50% -specifier=customRanged

Handle (custom) bool controls:

betterdisplaycli get n=DISPLAYNAME -control -specifier=customToggle
betterdisplaycli toggle n=DISPLAYNAME -control -specifier=customToggle
betterdisplaycli set n=DISPLAYNAME -control=off -specifier=customToggle
betterdisplaycli set n=DISPLAYNAME -control=on -specifier=customToggle
betterdisplaycli feed n=DISPLAYNAME -control=off -specifier=customToggle
betterdisplaycli feed n=DISPLAYNAME -control=on -specifier=customToggle

Command controls (first one invokes the defined default parameter, the second one provides an explicit parameter).

betterdisplaycli perform get n=DISPLAYNAME -control -specifier=customCommand 
betterdisplaycli perform get n=DISPLAYNAME -control=4 -specifier=customCommand