[processing] Add crs selection panel to scripts

[p0cisk]

CRS selection panel can be added as widget from scripts:

Simple type 'crs' as parametr type:

##outCrs=crs

Default CRS is WGS84, but it can be changed by passing optional value which can be any of string definition supported by QgsCoordinateReferenceSystem.:

##outCrs=crs EPSG:2180
##outCrs=crs proj4:+proj=tmerc +lat_0=0 +lon_0=19 +k=0.9993 +x_0=500000 +y_0=-5300000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

[mach0]

@volaya @alexbruy