Make Accelerator a standalone JS type.

This makes menu and global-shortcut share the same code on accelerator.

docs/api/accelerator.md

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+# Accelerator
+
+An accelerator is string that represents a keyboard shortcut, it can contain
+multiple modifiers and key codes, combined by the `+` character.
+
+Examples:
+
+* `Command+A`
+* `Ctrl+Shift+Z`
+
+## Platform notice
+
+On Linux and Windows, the `Command` key would not have any effect, you can
+use `CommandOrControl` which represents `Command` on OS X and `Control` on
+Linux and Windows to define some accelerators.
+
+## Available modifiers
+
+* `Command` (or `Cmd` for short)
+* `Control` (or `Ctrl` for short)
+* `CommandOrControl` (or `CmdOrCtrl` for short)
+* `Alt`
+* `Shift`
+
+## Available key codes
+
+* `0` to `9`
+* `A` to `Z`
+* `F1` to `F24`
+* Punctuations like `~`, `!`, `@`, `#`, `$`, etc.
+* `Space`
+* `Backspace`
+* `Delete`
+* `Insert`
+* `Return` (or `Enter` as alias)
+* `Up`, `Down`, `Left` and `Right`
+* `Home` and `End`
+* `PageUp` and `PageDown`
+* `Escape` (or `Esc` for short)
+* `VolumeUp`, `VolumeDown` and `VolumeMute`
+* `MediaNextTrack`, `MediaPreviousTrack`, `MediaStop` and `MediaPlayPause`