Implement consistent primary selection handling for X11

Hello,

On my Debian Linux, with KiCad 5.0.0, I experience the following bug:

When selecting a numerical value with the mouse (typically from a property window), the value fills the "clipboard buffer", but at the same time, the "primary selection" is emptied. The latter shouldn't happen and the "primary selection" should equally be filled with the value. As a consequence of this bug, the numerical values can not be pasted with the middle mouse button in other applications.

Can you reproduce?

Best regards,

Olivier

Example 1:
In Eeschema, open the property dialog of a label, and copy the content of the filed "Size".
Result:
$ xclip -selection primary -o
Error: target STRING not available # <-- THIS IS A PROBLEM
$ xclip -selection clipboard -o
1.5

Example 2:
In Pcbnew, open the property dialog of a footprint, and copy the content of the field "Width"
Result:
$ xclip -selection primary -o
Error: target STRING not available # <-- THIS IS A PROBLEM
$ xclip -selection clipboard -o
1

Note:
The bug does not happen with text values. Only with numerical values.

---
Application: kicad
Version: 5.0.0+dfsg1-2, release build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.60.0 OpenSSL/1.1.0e zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) libssh2/1.7.0 nghttp2/1.27.0 librtmp/2.3
Platform: Linux 4.14.0-3-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.62.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.61.0
    Compiler: GCC 8.2.0 with C++ ABI 1013

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_WXPYTHON=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=OFF