Rework addition of Symbian splash screen to reduce the source impact (uses SVG from Bug 2414) Notes: by using the OPTION SOURCEDIR parameter in the mifconv extension instructions, I can arrange to use the same source file name in sfimage, without having to export over the original Nokia file. This means that the name inside splashscreen.mbg is the same, which removes the need for the conditional compilation in SplashScreen.cpp, and gets rid of sf_splashscreen.mmp.

- AT Command plugin implements AT commands for AT+CKPD, AT+CBKLT, AT+CTSA
- This plugin will not be part of SF and will be moved to ext folder later
- To test this plugin we need to manually build 
- Optionally, after building we can generate a sisx file and use it for testing. Look at ATCmdPlugin\sis folder for information