Allow switching icon theme in musredit

Added the needed facility in the preferences which allows to switch
the icon scheme between plain and dark. This is needed for some OS
where qt doesn't provide the necessary information.

src/musredit_qt5/musredit/PAdmin.h

@@ -127,6 +127,7 @@ class PAdmin : public QObject
     bool    getDumpRootFlag() { return fDumpRoot; }
     bool    getEstimateN0Flag() { return fEstimateN0; }
     bool    getChisqPerRunBlockFlag() { return fChisqPreRunBlock; }
+    bool    getDarkThemeIconsFlag() { return fDarkThemeIcons; }
     QString getBeamline() { return fBeamline; }
     QString getInstitute() { return fInstitute; }
     QString getFileFormat() { return fFileFormat; }
@@ -150,6 +151,7 @@ class PAdmin : public QObject
     void setDumpRootFlag(const bool flag) { fDumpRoot = flag; }
     void setEstimateN0Flag(const bool flag) { fEstimateN0 = flag; }
     void setChisqPerRunBlockFlag(const bool flag) { fChisqPreRunBlock = flag; }
+    void setDarkThemeIconsFlag(const bool flag) { fDarkThemeIcons = flag; }
 
     void setFontName(const QString str) { fFontName = str; }
     void setFontSize(const int ival) { fFontSize = ival; }
@@ -193,6 +195,7 @@ class PAdmin : public QObject
     bool fChisqPreRunBlock;    ///< flag indicating if musrfit shall write 'per run block' chisq to the msr-file (default: no).
     bool fEstimateN0;          ///< flag indicating if musrfit shall estimate N0 for single histogram fits (default: yes).
     bool fEnableMusrT0;        ///< flag indicating if musrT0 shall be enabled at startup from within musredit (default: yes).
+    bool fDarkThemeIcons;      ///< flag indicating if dark theme icons shall be used (default: no)
 
     QString fBeamline;   ///< name of the beamline. Used to generate default run header lines.
     QString fInstitute;  ///< name of the institute. Used to generate default run header lines.