Add composer settings to QGIS options

This request moves a bunch of composer related settings which apply globally (ie, not just to the current composition) to a new "Composer" page in the qgis options window. These include the grid color and style, and also new options for the default font for composer items and the default grid and guide parameters. This helps users distinguish between settings which only apply to the current composer and settings which apply to all compositions, and makes room for a lot more composer default options (I'm thinking default page size and orientation for a start, and default composer units in the future).

I've also added a method for opening the options window at a specific page, and there's a shortcut to go straight the the composer preferences from composer windows.

One important thing to note is that I've moved the location of the grid settings, so previous settings for grid colour and style are ignored. There's two reasons I've done this:
1. To keep the QGIS settings organised and group all composer related settings together
2. The old QGIS default grid colour was solid black, no transparency. This is very visually heavy. I've set a new lighter, semi-transparent default color for the grid. Moving the grid settings to a new location means that all QGIS users will get this improved grid colouring by default.