Remove hard-coded 'Back' label button

Fixes Clang static analyzer warning:

file-utils.c:453:6: warning: Out of bound memory access (access exceeds upper limit of memory block)
        if (file_name [last_char] == G_DIR_SEPARATOR)
            ^~~~~~~~~~~~~~~~~~~~~

Fixes Clang static analyzer warning:

actions.c:137:5: warning: Use of memory after it is freed
    return full_uri;
    ^~~~~~~~~~~~~~~

Use 'GString' instead 'g_strconcat'

Use 'g_strlcpy' instead of 'strcpy', and 'g_strlcat' instead of 'strcat'

Fixes Clang static analyzer warnings:

fr-process.c:696:5: warning: Call to function 'strcpy' is insecure as it does not provide bounding of the memory buffer. Replace unbounded copy functions with analogous functions that support length arguments such as 'strlcpy'. CWE-119
                                strcpy(rarfile, argv[2]);
                                ^~~~~~

fr-process.c:698:5: warning: Call to function 'strcat' is insecure as it does not provide bounding of the memory buffer. Replace unbounded copy functions with analogous functions that support length arguments such as 'strlcat'. CWE-119
                                strcat(rarfile, "part1.rar");
                                ^~~~~~

fr-process.c:705:32: warning: Out of bound memory access (accessed memory precedes memory block)
                                        rarfile[strlen(rarfile)-5]=0;
                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~^~

Fixes Clang static analyzer warning:

fr-process.c:740:3: warning: Potential leak of memory pointed to by 'argv'
                g_warning ("The files could not be move: %s\n", commandline);
                ^~~~~~~~~