website: fix link to fityk-manual.pdf
SplitVoigt: change hwhm1/hwhm2 parameters to gwidth1/gwidth2

(the same names as in Voigt, hwhm* were there by mistake)
doc/model.rst: add a few words
gui: wxSOLID -> wxBRUSHSTYLE_SOLID
added Data::verify_options()

that warns if invalid option is specifed when loading data