UI: Add pause support

Adds support for pausing recordings.  When settings are eligible for
recordings, a pause button will appear next to the recording button.  If
the settings are not eligible, it will warn the user in the output
settings that they cannot pause recordings if those settings are used.

UI/CMakeLists.txt

@@ -235,6 +235,7 @@ set(obs_SOURCES
 	slider-ignorewheel.cpp
 	combobox-ignorewheel.cpp
 	spinbox-ignorewheel.cpp
+	record-button.cpp
 	volume-control.cpp
 	adv-audio-control.cpp
 	item-widget-helpers.cpp
@@ -289,6 +290,7 @@ set(obs_HEADERS
 	focus-list.hpp
 	menu-button.hpp
 	mute-checkbox.hpp
+	record-button.hpp
 	volume-control.hpp
 	adv-audio-control.hpp
 	item-widget-helpers.hpp