sym-lib-table: Symbols not loaded after "Append schematic"

When a schematics file is appended using "File->Append Schematic Sheet" its symbols are not displayed/loaded until eeschema is restarted or the symbols properties dialog is opened and closed for each symbol.

Steps to reproduce:

1. Open demo project 'pic_programmer' and remap symbols.
2. Create a hierarchical sheet 'test' in the root sheet and open it.
3. Use "File->Append Schematic Sheet" to append "pic_sockets.sch"
 -> all symbols are displayed as "??".

Pressing 'e' over a symbol to open the properties dialog and closing the dialog right afterwards will load the symbol.

Application: kicad
Version: (2017-11-12 revision a4b962e7d)-master, release build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.8 libidn/1.32 librtmp/2.3
Platform: Linux 4.10.0-38-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.58.0
    Curl: 7.47.0
    Compiler: GCC 5.4.0 with C++ ABI 1009

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