Update Linux to CEF 3.1453.1255

This merges @pritambaral's work from his fork of brackets-shell into our mainline. There's plenty of unfinished work (some already stubbed out, some not), but the current state of the shell is stable enough to be usable day-to-day. Even with some features (e.g. node and native menus) not implemented yet, the majority of Brackets is functional.

See https://github.com/adobe/brackets/wiki/Linux-Development-for-Contributors for setup instructions to install completely from scratch, i.e. download git repositories. 

If you already have the git repositories locally, follow these instructions

```
cd /path/to/brackets-shell

# build it
./scripts/setup_linux_build.sh
make

# checkout the linux branch in the brackets repo (this will be obsolete once the linux branch lands in the brackets repo master)
cd /path/to/brackets
git checkout linux
git submodule update --init --recursive

# package files
cd /path/to/brackets-shell
./installer/linux/stage_for_packaging.sh

# symlink to repo
ln -s /path/to/brackets /path/to/brackets-shell/installer/linux/staging/dev

# run brackets
./installer/linux/staging/Brackets
```