Add additional video player keyboard shortcuts (#275)

*  Skip forward 10 seconds ((CMD OR CTRL) ALT right)
 Skip back 10 seconds ((CMD OR CTRL) ALT left)
 Increase video speed ((CMD OR CTRL) +)
 Decrease video speed ((CMD OR CTRL) -)

* Codestyle fix

* The 'steps' should be implemented in base2, standard players use 1x, 2x, 4x, 8x, 16x

fixed bug with shift + "=" which is "+"

* resolve conflicts

* remove ide specific data
make playback rate more granular
add to menu skip and speed entries

* intendation fix

* conflict resolve

* rename setPlaybackRate to changePlaybackRate
setRate return boolean depending on whether this cast target supports setting the playback rate.
if setRate returns false - don`t change state
redundant else if statement in changePlaybackRate function

renderer/state.js

@@ -80,6 +80,7 @@ function getDefaultPlayState () {
     isStalled: false,
     lastTimeUpdate: 0, /* Unix time in ms */
     mouseStationarySince: 0, /* Unix time in ms */
+    playbackRate: 1,
     subtitles: {
       tracks: [], /* subtitle tracks, each {label, language, ...} */
       selectedIndex: -1, /* current subtitle track */