Canvas properties dialog - Image Size / Resolution Ratio
    
    - synfig-core - renddesc
    Add LINK_RES flag for resolution ratio
    Add x_res_ratio_ and y_res_ratio_ members to record current res ratio
    Add set_res_ratio( ) x and y ratio resolution setter
    Update set_x_res and set_y_res for LINK_RES flag and cyrrent res ratio
    
    - synfig-studio - renddesc
    Add Widget_Link toggle_res_ratio member and toggled signal