eeschema: Create hierarchical sheet: File extension not appended

When a new hierarchical sheet is created and no file extension (.sch) is specified in "Schematic Sheet Properties" dialog, a schematic file is created with the ".sch" extension but the new file is added to its parent sheet without file extension. This file cannot be opened if the project is (re-)loaded.

Steps to reproduce:
1. Open kicad control window and create a new project "test1"
2. Open schematics root sheet and add a new hierarchical sheet:
   - Use "power" as file name (not "power.sch")
   - "Power" as sheet name
   and press the "OK" button.
3. Close eeschema
4. (Re-)Open eeschema from control window
   -> Dialog with error message appears: "The entire schematic could not be loaded. Errors occurred attempting to load hierarchical sheet schematics.".

Remedy: Edit test1.sch and change line
  F1 "power" 50
into
  F1 "power.sch" 50

Application: kicad
Version: (5.0-dev-4084-gc02680cbf), release build
Libraries:
    wxWidgets 3.0.3
    libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.8 libidn/1.32 librtmp/2.3
Platform: Linux 4.13.0-32-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.3 (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