eeschema: Hierarchical sheet text fields clutters up when changing line thickness preferences

[endofexclusive]

This relates to how Eeschema displays the fields "File name" and "Sheet name" on a hierarchical sheet symmbol. Plotting the schematic does not have these issues.

Steps to reproduce:
1. Create a hierarchical sheet
2. Set the sheet field sizes for "File name" and "Sheet name" to 0.05 inch
3. Change line thickness in the menu "Preferences->General options" to 14

What is happening:
- The characters in the text start to grow into each other.
- The text fields collide with the sheet symbol outline.

What was expected:
It was expected that the line thickness preference should not affect the sheet text. Or possibly affect the text in a non-obfuscating manner.

Two patches have been prepared. The first patch clamps the text field thickness similar to how the plot operation does. The second patch improves calculation of the text field placement, taking line widths and text sizes into account.

Application: kicad
Version: (5.0.0-rc2-193-gdd436b658), release build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.60.0 OpenSSL/1.0.2k zlib/1.2.11 nghttp2/1.31.1
Platform: FreeBSD 11.1-RELEASE amd64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.66.0
    Curl: 7.60.0
    Compiler: Clang 4.0.0 with C++ ABI 1002

[Jeff Young (jeyjey)]

I have applied these to my 5.1 tree. Thanks for your contribution!